This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. Primary support for the subproject and the subproject's principal investigator may have been provided by other sources, including other NIH sources. The Total Cost listed for the subproject likely represents the estimated amount of Center infrastructure utilized by the subproject, not direct funding provided by the NCRR grant to the subproject or subproject staff. Fe K-edge x-ray absorption data will be measured on single crystals of hemoglobin (Hb), which is the ubiquitous di-oxygen transfer protein in mammals. Single crystal XAS data will be measured for deoxy-, met- and oxyHb, which are the reduced, fully oxidized and oxygenated states of Hb, respectively. XAS pre-edge analysis will be carried out to correlate with theoretically generated XAS spectra (using density functional theory) to determine the ground state electronic structure of oxyhemoglobin and understand the mechanism of di-oxygen binding to heme proteins. The data will be compared to solution Fe K-edge XAS data in order to compare the electronic structure changes between solution and crystalline forms of Hb. The data will be compared to those obtained from heme model complexes that are in the S=0, S=1 and S=2 states to investigate effects of spin state changes on the Fe K-edge shifts.